1. Field of the Invention
The present invention relates to a liquid crystal display (LCD), and more particularly, to an LCD comprising an optical uniaxial phase compensating film.
2. Description of the Prior Art
Owing to their low-profile, thin, and lightweight features, LCDs have become the mainstream display devices in recent years. Liquid crystal screens are widely used in electronic devices such as cellphones, personal digital assistants (PDAs), digital cameras, computers, notebooks, etc.
An LCD comprises an LC cell. An alignment of LC molecules in the LC cell is determined by variation of an electric field applied on the LC cell, and the transmission of light in the LC cell is adjusted accordingly. An LC material has a property of birefringence, which means that the refractive index of light in the direction of the long axis of the molecules is different from the refractive index of light in the direction of the short axis of the molecules. Therefore, linearly polarized incident light has diverse phases through different paths in the LC cell in the polarized direction. The feature of color and the transmission of light at a slant viewing angle are different from those at a front viewing angle.
Birefringence index of the LC molecules in the LC cell varies with an observation inclination. With the observation inclination increases, both of the contrast ratio of an image and image resolution decrease. To enhance the contrast ratio of the image obviously at a specific viewing angle and to reduce leakage of light in dark state on the LCD, a compensating film is attached to the LC panel of a conventional LCD. The birefringence of the LC molecules can be symmetrically compensated because the retardation value of light in different directions is compensated using the compensating film.
Please refer to FIG. 1 and FIG. 2, FIG. 1 shows a simulation of a distribution of light leakage in dark state after being compensated by a conventional uniaxial retardation film. FIG. 2 shows a simulation of a distribution of contrast over all viewing angles after being compensated by the conventional uniaxial retardation film. The optical path difference of liquid crystal Δn×d is set at 315 nm. The retardation values Ro and Rth of the A plate retardation film are 58 nm and 220 nm, respectively, and the retardation value Rth of the C plate is 40 nm. As can be seen from FIG. 1 and FIG. 2, under these circumstances there is severe light leakage problem in a horizontal viewing area. Generally, since the horizontal viewing area at is more visible than a vertical viewing area, contrast ratio and clarity in the horizontal viewing area affects viewing quality for observers than the vertical viewing area.
As a result, it is necessary to restrict the area of leakage of light in dark state within the vertical viewing area, rather than the horizontal viewing area.